i was reading a blog and apparently the main movie follows the original plot of the book but they also took some liberties and tell some of the history behind gandalf and other main characters that were not covered in the book.
I read Lord of the Rings at least 3 times and was very sceptical that they would do a good job on the movies but I thought they were very well done and really enjoyed them. I hope the Hobbit comes out just as good.